Studies are being carried out on a glycolipoprotein from platelets, erythrocytes, brain and liver which binds serotonin. The binding is irreversible and biochemically specific in that compounds such as nonradioactive serotonin, 5-hydroxyindole, 5-hydroxyindole acetic acid and reserpine can block uptake. On the other hand, indole, indole acetic acid, and tryptophane do not block binding. Since binding is retarded by antioxidants or anaerobic conditions, an oxidative phenomenon appears to be involved. This extract may represent a biochemical framework for the recognition of indole moieties. Studies on this extract are continuing.